The present invention relates to an electrostatic spray gun for coating material and, more particularly to an electrostatic spray gun having an actuating lever for controlling both the flow of coating material and the electrostatic charging of coating material.
Electrostatic spray guns typically employ an electrode for applying an electrostatic charge to coating material as it flows through the spray gun and exits from an atomizer nozzle of the spray gun. In certain circumstances, it is desirable to atomize the coating material without applying an electrostatic charge. For example, when coating a narrow cavity, the application of an electrostatic charge would cause much of the coating material to be deflected laterally because the electric field would not penetrate into the cavity. As a result, most of the coating material could concentrate on the outer edges of the cavity and only a small amount of coating material would enter into the cavity. It is, therefore, desirable to be able to control the electrostatic charging of the coating material.
Typically, the control of electrostatic charge in spray guns is effected by a magnetically actuatable electric switch which can connect or disconnect an electric power supply to the electrode. Magnetically actuatable electric switches consist of so-called "reed" switches in which a pair of electrical contacts are contained in a small glass tube and can be opened or closed by the application of an external magnetic field. A magnetically actuatable switch is necessary for electrostatic spray guns because many coating materials are explosive and could be ignited by the sparks generated by the opening and closing of an ordinary switch.
Electrostatic spray guns often employ a single lever to control both the flow of coating material through the spray gun as well as the application of electrostatic charge to the coating material. In such spray guns, the coating material supply line is opened and the electric power supply to the electrode is connected, either simultaneously or in succession, as the actuating lever is moved. In the latter case, when the connection is effected in succession, the spray gun is often designed so that partial movement of the actuating lever will actuate the spraying of coating material, but not the magnetically actuatable electric switch, so that the electrode is not connected to the electric power supply, and the sprayed coating material is not electrostatically charged. In such a device, the operator must approximate how far to move the lever in order to actuate spraying of the coating material without connecting the high voltage to the electrode. The operator must therefore be skillful and experienced. Moreover, the fine touch required by this design can become quite tedious for even the most skilled operator.
The spray gun disclosed in U.S. Pat. No. 4,441,656 overcomes this problem. In this device, a first magnet is employed for producing a magnetic field to actuate a magnetically actuatable electric switch, and a second magnet is employed to produce a second magnetic field which, by appropriate adjustment of the second magnet, can be superimposed over the first magnetic field so that the switch is continuously open, i.e. so that the electrode is not connected to high voltage regardless of the position of the actuating lever and the corresponding position of the first magnet. The second magnet of this device can be adjusted independently of the position of the actuating lever.
The energy supply circuit of electrostatic spray guns typically consists of a high-voltage transformer coupled to a high-voltage multiplier circuit. Such an arrangement is disclosed, for example, in U.S. Pat. No. 3,608,823. The magnetically actuatable electric switch is located in the electric supply line on the primary side of the transformer.